High altitude aircraft require oxygen enriched air either as emergency backup in the event of loss of cabin pressure as in commercial transports or as an on-line system which controls oxygen enrichment as a function of altitude and other parameters as in military aircraft. Oxygen enrichment can be achieved using oxygen sources such as stored liquid oxygen, high pressure oxygen gas, oxygen generators, sometimes referred to as candles, or fractionalized air. Except in the case of fractionalizing air, the oxygen source represents a discreet quantity limited by storage capacity and/or weight which can be critical in airborne applications. Air fractionalizing is a continuous process, and, thus, represents advantages where capacity, supply logistics, or weight are problems.
Air fractionalizing is normally accomplished by alternating the flow of high pressure air through each of two beds containing a molecular sieve material such as zeolite. This process is identified as the pressure swing adsorption technique and it employs a myriad components, mechanical, electrical and pneumatic. Though highly reliable, the number of components making up a pressure swing system suggests the probability of an intermittent failure. In high altitude military aircraft, where a single such failure could be catastrophic, it is very desirable to maintain a backup system usually comprised of high pressure oxygen bottles. This high pressure gas can also be used at very high altitudes to achieve oxygen concentrations above those attainable by pressure swing adsorption systems due to the trace gases such as argon which are not adsorbed and exit the adsorption system as part of the product gas.
In an aircraft using an air fractionalizing oxygen enriching system with high pressure bottled oxygen backup, various modes of operation of the two systems in combination are possible. These modes include operation from the bottled gas, from the fractionalized air, or an automatic mode in which either of the two sources is selected based on altitude, oxygen concentration in the breathing system and/or breathing system pressure.